1. Field of Invention
The present invention relates to ink jet printers and more particularly relates to controlling spot sizes by applying prepulses to a printhead.
2. Description of Related Art
A thermal ink jet printhead selectively ejects droplets of ink from a plurality of drop emitters to create a desired image on an image receiving member, such as a sheet of paper. The printhead typically comprises an array of the drop emitters that convey ink to the image receiving member. In a carriage-type ink jet printhead, the printhead moves back and forth relative to the image receiving member to print the image in swaths. Alternatively, the array may extend across the entire width of the image receiving member to form a full-width printhead. Full-width printheads remain stationary as the image receiving member moves in a direction substantially perpendicular to the array of drop emitters.
An ink jet printhead typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink and a portion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink (an ink droplet or a main ink droplet and smaller satellite drops) to be ejected from the emitter to the image receiving member. U.S. Pat. No. 4,774,530 to Hawkins, the disclosure of which is incorporated herein by reference, shows a general configuration of a typical ink jet printhead.
When a quantity of ink in the form of a droplet is ejected from the ejector to a copy surface, the resulting spot becomes part of a desired image. Uniformity in spot size of a large number of droplets is very important for image quantity. If the volume of droplets ejected from the printhead over the course of producing a single document are permitted to vary widely, the lack of uniformity will have noticeable effects on the quality of the image. Similarly, if volumes of droplets ejected from the printhead differ during subsequent printings of the same document, then printing stability cannot be maintained; this is particularly important in color printing. The most common and important cause of variance in the volume of droplets ejected from the printhead is variations in the temperature of the printhead over the course of use. The temperature of the liquid ink, before vaporization by the heating element, substantially affects both the nucleation behavior and the viscosity of the ink. These two properties substantially influence the resulting spot size on the copy surface. Control of temperature of the printhead has long been of primary concerns in the art.
In order to maintain a constant spot size from an ink jet printhead, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., the disclosure of which is incorporated herein by reference. In this patent the printhead has a number of heater resistors and a temperature sensor which operate to heat the printhead to an optimum operating temperature and maintain that temperature regardless of local temperature variations.
U.S. Pat. No. 4,791,435 to Smith et al. the disclosure of which is incorporated herein by reference, discloses an ink jet system wherein the temperature of the printhead is maintained by using the heating elements of the printhead not only for ejection of ink but for maintaining the temperature as well. The printhead temperature is compared to thermal models of the printhead to provide information for controlling the printhead temperature. At low temperature, low energy pulses are sent to each channel, or nozzle, below the voltage threshold which would cause a drop of ink to be ejected. Alternatively, the printhead is warmed by firing some droplets of ink into an external chamber or "spittoon," as opposed to the copy surface.
PCT Application 90/10541, the disclosure of which is incorporated herein by reference, describes a printhead in which the heating cycle for the ink is divided into several partial cycles, only the last of which initiates bubble formation and ejection of a droplet. In this printhead, therefore, the liquid ink is first preheated to a preselected temperature, wherein the ink will have known volume and viscosity characteristics so that the behavior of the ink will be predictable at the time of firing.
PCT Application 90/10540, the disclosure of which is incorporated herein by reference, discloses a printhead control system wherein the temperature of the liquid ink is compared with a predetermined threshold value, and if it exceeds this threshold value, the pulse energy (proportional to the square of the voltage to the heating element times the time duration of the pulse) is reduced. According to this patent, the pulse energy may be varied by controlling either the voltage, the pulse duration, or both.
U.S. Pat. No. 4,736,089 to Hair et al., the disclosure of which is incorporated herein by reference, discloses a thermal printhead (as opposed to an ink-jet printhead) wherein the printhead temperature is sensed by a voltage generating diode on the printhead itself A detected temperature of the printhead is used to establish a preselected reference level. Bi-stable means are coupled to the thermal printhead to print or not print at a given time. Control means are used to turn the bi-stable means on when the controlled voltage is less than the reference level related to the temperature, and turn the bi-stable means off when the controlled voltage exceeds the preselected reference level, thus causing the time duration of a voltage pulse to the thermal printing means to be dependent on temperature.
U.S. Pat. No. 4,980,702 to Kneezel, the disclosure of which is incorporated herein by reference, discloses a thermal ink jet printhead wherein outputs from a temperature sensor in the printhead are compared to a high or low level temperature reference. If the sensed printhead temperature is below the reference value, power to the heater in the printhead is turned on. If the temperature sensed is too high, the heater is turned off. The printhead is configured so that the temperature sensor and heater in the printhead are in close proximity.
U.S. Pat. No. 4,982,199 to Dunn, the disclosure of which is incorporated herein by reference, discloses a method and apparatus for gray scale printing with a thermal ink pen. A firing resistor is driven by a plurality of pulses to eject a droplet of ink from a nozzle. Prewarming of the ink in the firing chamber is achieved by applying an electrical warming pulse signal to the resistor prior to a firing pulse signal. The firing pulse signal causes the drop to be ejected. The warming pulse may be a plurality of pulses applied sequentially prior to the firing pulse which transfers a desired quantity of thermal energy to the ink. The prewarming of the ink by the warming pulse or pulses increases the volume of the ink droplet. By varying the degree of prewarming the droplets ejected by the firing pulse can be varied in volume, yielding gray scale printing.
European Patent Application No. 0 496 525 A1, the disclosure of which is incorporated herein by reference, discloses an ink jet recording method and apparatus in which ink is ejected by thermal energy produced by a heat generating element of a recording head. According to one aspect, driving means apply plural driving signals to the heat generating element for every ink droplet ejected. The plural driving signals include a first driving signal for increasing a temperature of the ink adjacent the heater without creating the bubble, and a second driving signal subsequent to the first driving signal with an interval therebetween, for ejecting the ink. Additionally, a width of the first driving signal is adjustable so as to change an amount of the ejected ink.
European Patent Application No. 0 505 154 A2, the disclosure of which is incorporated herein by reference, discloses a thermal ink jet recording method and apparatus which controls an ink ejection quantity by changing driving signals supplied to the recording head on the basis of a variation in temperature of the recording head. A preheat pulse is applied to the ink for controlling ink temperature and is set to a value which does not cause a bubble forming phenomenon in the ink. After a predetermined time interval, a main heat pulse is applied which forms a bubble in the ink to cause ejection of a droplet (or a main droplet and satellite drops) of ink from an ejection port.
U.S. patent application No. 08/220,720 to Stephany, the disclosure of which is incorporated herein by reference, discloses a power control system for a printer which has at least one heating element for producing spots. The system includes a thermistor disposed on a printhead which senses the temperature of the printhead. The sensed temperature is used to vary pulses applied to the at least one heating element to maintain a constant spot size.